1. Field of the Invention
The present invention relates to a steam turbine blade and a method for manufacturing the steam turbine blade, particularly which can maintain and develop the aerodynamic characteristics of the rotor blades (blades) and the stator blades (nozzles) composing the steam turbine and thus the performance of the steam turbine.
2. Background of the Invention
In a steam turbine, the pressure and high temperature energy of the high temperature and high pressure steam supplied from the boiler is converted into the corresponding rotational energy using the turbine cascade of the rotor blades and the stator blades. FIG. 3 is a conceptual view about a power generating system using such a steam turbine.
As shown in FIG. 3, a steam generated at a boiler 1 is heated again at a superheater 2 and then supplied to a steam turbine 3.
The steam turbine 3 is configured so as to have a plurality of turbine stages which are arranged in the axial direction of a turbine rotor 4, each turbine stage being constituted from rotor blades implanted in the turbine rotor 4 along the circumferential direction thereof and stator blades (nozzles) supported by a casing. Then, the steam supplied to the steam turbine 3 is expanded in the steam path so that the high temperature and high pressure energy is converted into the rotational energy at the turbine rotor 4.
The rotational energy of the turbine rotor 4 is transmitted to a turbine generator 9 connected with the turbine rotor 4 and thus converted into the corresponding electric energy. On the other hand, the steam, from which the high temperature and high pressure energy is extracted, is discharged from the steam turbine 3 and supplied to a steam condenser 10 so as to be cooled down by a cooling medium 11 such as seawater and then converted into the corresponding condensed water. The condensed water is supplied again to the boiler 1 by a feed pump 12.
By the way, the steam turbine 3 is divided into a high pressure turbine, an intermediate pressure turbine and a low pressure turbine commensurate with the temperature and pressure condition of the steam to be supplied. In such a power generating system, since the stages of the high pressure turbine and the intermediate pressure turbine suffer from the high temperature condition, the rotor blades and stator blades of the stages of the high pressure turbine and the intermediate pressure turbine may be oxidized remarkably.
When the rotor blades and the stator blades are incorporated as parts of the steam turbine, the surface roughness of the rotor blades and the stator blades are reduced as possible by blowing minute particles off onto the surfaces of the rotor blades and the stator blades because the flow of a fluid fluctuates on the surfaces of the rotor blades and the stator blades and thus separate from the surfaces thereof so as to lower the aerodynamic characteristics of the rotor blades and the stator blades and deteriorate the turbine efficiency entirely if the surface roughness of the rotor blades and the stator blades is enlarged.
Such a problem is pointed out as the rotor blades and the stator blades can exhibit excellent aerodynamic characteristics at the initial stage because the surface roughness of the rotor blades and the stator blades is small, but cannot exhibit the excellent aerodynamic characteristics with the operation period of time because the surfaces of the rotor blades and the stator blades are oxidized gradually to coarsen the surface roughness of the rotor blades and the stator blades and then to deteriorate the aerodynamic characteristics thereof, resulting in the deterioration of the entire turbine efficiency. The techniques relating to the above-described problem are proposed as below.
In order to enhance the corrosion-resistance, oxidation-resistance and fatigue strength of the steam turbine parts, it is proposed that a nitrided hard layer (radical nitrided layer) is formed on the steam turbine parts and then a physical evaporation hard layer made of, e.g., CrN, TiN, AlCrN is formed thereon (refer to Reference 1).
Moreover, nickel plating is conducted for a high temperature member for the steam turbine rotors so that the thus plated member is borided to form a layer made of iron boride and nickel boride at the surfaces of the steam turbine rotors, thereby enhancing the corrosion-resistance and the high temperature erosion-resistance of the steam turbine rotors (refer to Reference 2).
Furthermore, a Cr23C6 layer is formed at the steam turbine blades by means of the combination of plating and thermal treatment so as to enhance the corrosion-resistance, wear-resistance and the erosion-resistance of the steam turbine blades (refer to References 3 and 4).
In addition, it is proposed that the corrosion-resistance of the steam turbine blades is enhanced by means of laser plating where a cobalt alloy with strictly controlled composition is contacted with a base material, and then melted and adhered with the base material by means of laser (refer to Reference 5).    [Reference 1] JP-A 2006-037212 (KOKAI)    [Reference 2] JP-A 2002-038281 (KOKAI)    [Reference 3] JP-A 08-074024 (KOKAI)    [Reference 4] JP-A 08-074025 (KOKAI)    [Reference 5] JP-A 2004-169176 (KOKAI)
However, the above-described conventional techniques require complicated processes, respectively, resulting in the increase of the manufacturing cost. Moreover, the conventional techniques enlarge the surface roughness of the steam turbine rotors through the formation of the layer, resulting in the inherent deterioration of the initial turbine performance. In this point of view, such a method as enhancing the oxidation-resistance of the steam turbine blades under the condition that the initial surface roughness of the steam turbine blades is not changed is not proposed as of now.